1. Field of the Invention
The present invention relates to an optical pick-up apparatus for recording and reading an optical disc, which is different in an substrate thickness and a wavelength to be used.
2. Description of the Prior Art
Conventionally, in an optical disc being used at present, there are variations such as a CD, a CD-ROM and a DVD, and higher density optical disc has been developed. These informations to be recorded on optical discs have been recorded with a bit on the recording surface of an optical disc by forming a pit or a mark corresponding to the pit, it is adapted to determine the existence if any information by illuminating a light beam on the surface recorded with information and detecting the reflected light beam strength.
According to the kind of an optical disc, there is a difference in the pit shape or the shape of mark corresponding to the pit, recording system and the thickness of the substrate, in the case where reading of different kinds of discs is carried out, a light beam in accordance with the wavelength due to the kind of disc has to be illuminated. For example, to a CD and a CD-ROM (hereinafter referred to xe2x80x9cstandard density optical discxe2x80x9d) having the substrate thickness of 1.2 mm, a light source to emit a light beam having a wavelength of 780 nm band should be applied, and to a DVD (hereinafter, referred to xe2x80x9chigh density optical disc) having the substrate thickness of 0.6 mm, a light source to emit a light beam having a wavelength of 635-650 nm band should be applied.
For that purpose, in order to read different kinds of optical discs with a single optical pick-up apparatus, such pick-up apparatus comprises light sources having different wavelengths corresponding to each optical disc and an objective lens to condense a light beam on the recording layer of each disc.
However, it is a main trend that an optical pick-up apparatus comprises such constitution that is made to condense the light beam on the recording surface of different kinds of optical discs with a single objective lens in order to make it small-sized, light-weighted and low-cost.
A general constitution of such kind of an optical pic-up apparatus is shown in FIG. 11 and explained below.
FIG. 11(a) shows a schematic drawing in the case where a high density optical disc 1a having a substrate thickness of 0.6 mm is read and FIG. 11(b) shows a schematic drawing in the case where a standard density optical disc 1b having a substrate thickness of 1.2 mm is read.
Hereinafter, a constitution of an optical pick-up apparatus is explained referring to FIG. 11(a).
On the light path of a light beam emitted from a first light source 2 corresponding to the high density optical disc 1a, from the close side to the first light source 2 are disposed, in turn, a wavelength selection mirror 3, a beam splitter 4, a collimator lens 5 and a raising mirror 6, wherein the raising mirror 6 is disposed in such a manner as it opposes to the recording surface of the high density optical disc 1a. Between the high density optical disc 1a and the raising mirror 6 an actuator movable portion 7 is disposed. Here, a two-dotted chain line shows an axis A which connects the first light source 2 and the raising mirror 6. And a position perpendicular to the axis A and opposing to the wavelength selection mirror 3, a second laser light source 8 corresponding to the standard density optical disc 1b is disposed. Further, at a position perpendicular to the axis A and opposing to the beam splitter 4, a wavelength selection mirror 9 and a cylindrical lens 10 are disposed, and at a position where the cylindrical lens 10 condenses on a first photo detector 11 which the light beam reflected on the recording surface of the high density optical disc 1a enters is disposed. And, at a position opposing to the wavelength mirror 9, a cylindrical lens 12 and a second photo detector 13 are disposed in parallel with the axis A.
The wavelength selection mirror 3 and the wavelength selection mirror 9 are adapted to transmit the light beam emitted from the first light source 2 and to reflect the light beam emitted from the second light source 8, and the beam splitter 4 transmits the light beams from the light sources 2 and 8, and reflects the signal light beams reflected on the optical discs 1a and 1b to the photo detector 11 and 13. The collimator lens 5 is adapted to convert the light beam into a parallel light beam, and the raising mirror 6 is adapted to reflect the light beam which is transmitted the collimator lens 5 toward the vertical direction and to enter the actuator movable portion 7 by changing the light progressing direction.
On the actuator movable portion 7, an objective lens 14 and an opening limiting plate 15 are mounted, and the objective lens 14 is designed to have the most suitable opening diameter when the high density optical disc 1a is read. Since the opening limiting plate 15 is adapted to transmit the light beam emitted from the first light source 2 and has an aperture formed with a wavelength filter film 16 which cuts off the light beam emitted from the second light souce 8, the diameter of the light beam emitted from the second light source 8 can be limited. Therefore, when this limited light beam is transmitted the objective lens 14, the aperture is designed to have the most suitable opening diameter to read the standard density optical disc 1b. 
And the cylindrical lenses 10 and 12 are rod-like lens, and give an astigmatism the light beam which enters the first photo detector 11 and the second photo detector 13. Further, the first photo detector 11 detects the signal light from the high density optical disc 1a and the second photo detector 13 detects the signal light from the standard density optical disc 1b. 
The thus-constituted optical pick-up apparatus of the high density optical disc 1a will be explained.
The light beam emitted from the first light source 2 is transmitted the wavelength selection mirror 3 and the beam splitter 4 and progresses in the collimator lens 5 and is converted into parallel light beams having an equally widened angle with each other. The converted parallel light beams progress toward the high density optical disc 1a with the raising mirror 6 and is transmitted the opening limiting plate 15 and are condensed most suitably on the recording surface of the high density optical disc 1a with the objective lens 14.
And, the light beam reflected on the recording surface of the high density optical disc 1a becomes a signal light and traces the same path reversely, which, however, is branched toward the photo detector 11 through the beam splitter 4. Further, by being transmitted the wavelength selection mirror 9 and the rear cylindrical lens 10, the light beam is given with the astigmatism to be detected with the first photo detector 11.
Next, the reading operation of the standard density optical disc 1b is explained referring to FIG. 11(b).
The light beam emitted from second light source 8 is reflected on the wavelength selection mirror 3, and is transmitted the beam splitter 4 and is converted into a parallel light beam with the collimator lens 5. Then, the parallel light beam progresses toward the standard density optical disc 1b with the raising mirror 6. Further, since the light beam is cut-off with the wavelength filter film 16 of the opening limiting plate 15, the light beam of only a central portion can be transmitted the opening limiting plate 15. The light beam thus limited in the beam diameter is condensed on the recording surface of the standard density optical disc 1b with the objective lens 14.
And, the light beam reflected on the recording surface of the standard density optical disc 1b becomes a signal light, traces the same path reversely and is branched toward the photo detector 11 with the beam splitter 4.
Further, the light beam is reflected with the wavelength selection mirror 9 to the direction of the second photo detector 13, to be given with the astigmatism and is detected with the second photo detector 13.
Now, in the above conventional optical pick-up apparatus, since, in order to read different kinds of high density optical disc 1a and the standard density optical disc 1b, it is carried out to control the opening diameter of the objective lens 14 with the opening limiting plate 15 of the actuator movable portion 7, the following problems are arisen.
That is, when the standard density optical disc 1b is read, the light beam is cut-off with the wavelength filter film 16 of the opening limiting plate 15 and a light beam of only a sectional central portion can be transmitted the opening limiting plate 15, merely thereby, a beam spot to be condensed on the recording surface of the standard density optical disc 1b is apt to generate a wave front aberration, which influences the reading property of the recorded information.
In addition, in the above conventional optical pick-up apparatus, a small-sizing and a light-weighting are realized by condensing the light beam on different kinds of optical discs through a single objective lens. However, since the opening limiting plate 15 is provided, the number of parts is increased. Further, since the assembling time is increased, an increase in cost of an apparatus is invited, and since the opening limiting plate 15 is provided at the actuator movable portion 7, the increase in weight of the actuator movable portion 7 is invited, the property of the actuator will be deteriorated.
Accordingly, the present invention is made in the light of the above problem, without using an opening limiting plate, the diameter of the either light beam of two kinds of light beams is limited and each light beam is condensed most suitably on the recording surface of different kinds of optical discs with only one objective lens to provide a small-sized and low-cost optical pick-up apparatus.
In order to attain the above object, according to a first aspect of the present invention, in order to read two different kinds of first and second optical discs being different in substrate thickness and wavelength to be used, an optical pick-up apparatus comprises light sources which generate light beams respectively corresponding to said two kinds of optical discs, and a photo detector to detect signal lights reflected on said two optical discs, wherein one condenser lens condensing the light beam emitted from the light source corresponding to the first optical disc most suitably on the first optical disc is disposed, and said condenser lens condenses the light beam emitted from the light source corresponding to the second optical disc most suitably on the second disc by limiting the diameter of the light beam emitted from the light source corresponding to the second optical disc, and an opening limiting portion which changes the beam diffusion angle of the light beam emitted from the light source corresponding to the second optical disc is disposed between said light sources and said condenser lens.
According to a second aspect of the present invention, in the first aspect, said opening limiting portion consists of plano-concave lens having a plane portion and a concave portion, which is set slantwise to a light axis at a given angle, and on the plane surface which is a incident surface of the light beam, an optical filter film which reflects the light beam corresponding to the first optical disc and transmits the light beam corresponding to the second optical disc is formed, and on the concave surface an all-reflecting film which limits the beam diameter of the light beam corresponding to the second optical disc which enters from the plane surface is formed.
According to a third aspect of the invention, in the first aspect, said opening limiting portion comprises a piano concave lens, a plane glass adhered to the plane side thereof, which is set slantwise to a light axis at a given angle, on the non-adhered surface of the plane glass which is a light incident surface, an optical filter film which reflects the light beam corresponding to the first optical disc and transmits the light beam corresponding to the second optical disc is formed, and on the adhered surface of the plano-concave lens, a light absorption film for limiting the beam diameter of the light beam corresponding to the second optical disc, which enters from the non-adhered surface of the plano-concave lens, on the concave surface of the non-adhered surface of the plano-concave lens, an all-reflecting film which reflects inside the beam entered.
According to a fourth aspect of the present invention, in the second or third aspect, an optical filter film of the opening limiting portion transmits or reflects the light beam in accordance with the wavelength of the light beam.
According to a fifth aspect of the present invention, in the second or third aspect, the optical filter film of the opening limiting portion transmits or reflects the light beam in accordance with a polarization direction.
The present invention, by the above constitution, in order to condense the light beam most suitably on the second optical disc, the opening limiting portion to limit the beam diameter of the light beam is disposed between the first optical disc and the light source, so that, when the first optical disc is read, the light beam is condensed most suitably on the disc with the condenser lens. On the other hand, when the second optical disc is read, since the beam diameter is limited with the opening limiting portion, the light beam, which is even what corresponds to the second optical disc, is condensed most suitably on the disc with the condenser lens. Further, by changing the beam diffusion angle of the light beam emitted from the light beam corresponding to the second optical disc, the generation of the wave front aberration is restrained.